Multi-Protocol Label Switching (MPLS) is a set of standards used in telecommunication networks for passing data packets from a node to other nodes. Each data packet has a label stack, on which forwarding decisions are made without the need to read or decode the rest of the packet.
Based on the existing MPLS forwarding plane, the Multi-Protocol Label Switching Transport Plane (MPLS-TP) aims to provide more functions relevant to transport level network requirements. One key area is the Operations, Administration and Maintenance (OAM) function set. This OAM function set is under standardization in the Internet Engineering Task Force (IETF). The OAM framework and requirements include functions such as Continuity Check (CC), Connectivity Verification (CV), Management Signals, etc.
MPLS-TP provides that the necessary OAM attributes (identifiers, timers, counters, etc.) are encoded in so called OAM frames that are in-band, that is multiplexed into the data flow. This means the OAM frames and data flow experience the same process in being forwarded from one node to another. The OAM frames are generated and received by Maintenance EndPoints (MEPs) at the endpoints of a connection; some of them (the on-demand) can be terminated at MIPs (Maintenance Intermediate Points). Currently, the MPLS-TP architecture consists of three layers: Pseudowire (PW), Label-Switched Path (LSP) Tunnel (connection) and Section (data link) layers. The MPLS-TP OAM can support all three layers.
Currently, several competing solutions of encoding the OAM parameters are proposed, though the same method of multiplexing the OAM frames into the data flows is used. An associated channel is defined to carry the OAM frames and other control/management frames. For PWs, these channels are identified and de-multiplexed using only the Generic Associated Channel Header (G-ACH). In the case of LSP tunnels and sections, an additional label, the Generic Association Channel Label (GAL) indicates the associated channels. At the egress side, the MEP detects and de-multiplexes the OAM frames based on these labels. However, if it is desired to include in a network different nodes with different and incompatible ways of handling OAM information exchange, then the paths will need to be terminated and the OAM information demultiplexed and transmitted separately to the incompatible node.